Ultrasound imaging operates by sending a number of short pulses of acoustic energy from a transducer into a region of interest and collecting the information contained in the corresponding echo signals. FIG. 1A shows a simplified ultrasound transducer having a number of individual transducer elements 12 (not drawn to scale) that vibrate and produce ultrasonic acoustic signals when a varying voltage is supplied across the elements. The elements also produce electronic signals when the elements receive acoustic energy. The elements 12 are typically arranged in a one or two-dimensional array that includes one or more matching layers 14 and a fixed lens 16. By carefully selecting the amplitude and the time at which the driving signals are applied to each of the transducer elements, the acoustic signals constructively combine to form a beam with a focal zone at a desired location. As the operating frequency of the transducer increases, the size of the focal zone (often the shape of a grain of rice) decreases. For example, at a 15 MHz center frequency, the size of the focal zone is about 500×300 μm. At 30 MHz, the size of the focal zone drops to approximately 280×150 μm. and at 50 MHz, the size of the focal zone is less than 200×100 μm. In addition to ultrasound arrays, ultrasound signals can also be generated by single-element transducers 17 as shown in FIG. 1B.
Ultra-high frequency (UHF) diagnostic ultrasound has progressed substantially in the past 10 years in both preclinical and clinical industries, with the introduction of systems with 50 MHz center frequency arrays having upper corner frequencies of over 70 MHz. There are many new scientific and medical possibilities that can be explored resulting from the higher resolution and bandwidth of UHF ultrasound, However, along with new applications and capabilities comes new testing and characterization challenges. As one skilled in the art will appreciate, as transducers push ever higher in frequency, wavelengths decrease accordingly, and various other mechanisms such as non-linear propagation of acoustic waves in water become more and more prevalent. There is currently a need to understand the character of UHF ultrasound in water both scientifically and for the purposes of regulation of medical and preclinical devices. In addition, to take advantage of modern sophisticated FEA modelling, there is a need to accurately measure acoustic fields at or even below the pitch of the array. There is clearly a need for smaller aperture hydrophones with higher frequency calibrations to ensure accurate measurement of harmonics and to reduce spatial uncertainties arising from short wavelength sound waves being measured with relatively large aperture hydrophones.
Before an ultrasound transducer can be approved for clinical use in the United States by the Food and Drug Administration (FDA) or can obtain the CE mark for clinical use in Europe, the acoustic energy produced by the transducer must be characterized. The characterization produces a map of the pressure intensities to make sure the focal zone is well defined and that the transducer is not producing hot spots of energy in undesired locations. Similarly, the characterization confirms that the energy produced is not so great that it will cause cavitation in tissue to be examined, and that power output is within acceptable limits imposed by various organizations. Well established standards exist to prescribe the testing protocols and results required for regulatory approval. However, UHF ultrasound has increasingly pushed these tests to the limits and beyond due to the lack of suitably small hydrophone aperture sizes and sufficiently high frequency calibration data.
As shown in FIG. 2, most transducer testing is performed by operating a transducer 20 in a liquid bath 40 (typically de-gassed water but could be another liquid). A hydrophone 50 is placed on a computer controlled stage (not shown) in the path of the ultrasound beam. As the transducer is operated, the stage is moved to cause the hydrophone to measure the location of the focal zone and the intensity of the beam at a number of locations. Signals from the hydrophone are stored by a computer system to confirm that the transducer is operating as intended. A plot of the intensity measurements in space defines the characteristics of the ultrasound transducer beam.
Membrane style hydrophones are the most desirable to use in sampling an ultrasound beam because of their flat frequency response and simple interactions with the radiation pattern created by the device under test (DUT). In order to be able to effectively sample the beam, the active area of the hydrophone must be substantially smaller than the focal zone of the transducer under examination. In the past, it has been difficult to reliably manufacture a membrane style hydrophone with a sufficiently small active area that can be used to test high frequency ultrasound transducers. Therefore, users have been forced to use needle-type hydrophones, which exhibit undesirable resonances and interactions with the radiation pattern being measured. In addition, specially shaped needle hydrophones that are designed to minimize unwanted resonances such as so called “lipstick style” hydrophones are used. However, in practice it is difficult to accurately manufacture such shapes to a small enough scale for very high frequency ultrasonic characterization. The result is that needle-type hydrophones are not as accurate in characterizing high frequency beam patterns as membrane style hydrophones.
Given these problems, there is a need for an improved high frequency membrane style hydrophone as well as a method for manufacturing such hydrophones.